Fuel element

ABSTRACT

THIS INVENTION PERTAINS TO AN IMPROVED METHOD FOR MANUFACTURING FUEL ELEMENTS FOR NUCLEAR REACTORS BY SURROUNDING THE FUEL ELEMENTS WITH AN ENVELOPE OF SYNTHETIC PLASTIC MATERIAL AND THEREAFTER APPLYING DEFORMATION FORCES TO THE SYNTHETIC PLASTIC ENVELOPE IN ORDER TO DE-   CREASE OR COMPLETELY REMOVE ANY GAPS EXISTING BETWEEN THE NUCLEAR FUEL MATERIAL AND ITS SURROUNDING CASING.

Dec. 14, 1971 L, .J. AERTS ETAL Re. 27,242

FUEL ELEMENT Original Filed March :5, 1967 FIG. 3.

FIG. 2.

FIG

FIG. 5.

FIG. 4.

United States Patent 11 Int. Cl. B23p 17/00, 9/00 U.S. Cl. 29-400 8Claims Matter enclosed in heavy brackets appears in the original patentbut forms no part of this reissue specification; matter printed initalics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE This invention pertains to an improved methodfor manufacturing fuel elements for nuclear reactors by surrounding thefuel elements with an envelope of synthetic plastic material andthereafter applying deformation forces to the synthetic plastic envelopein order todecrease or completely remove any gaps existing between thenuclear fuel material and its surrounding casing.

BACKGROUND A classical fuel element for nuclear reactors is generallycomposed of nuclear fuel used in any known form as for instance in theform of pellets, spheres, powders, or grains-surrounded by a metalliccanning or cladding.

It is known that such a canning often does not exactly fit the fuel,thus giving rise to one or more gaps which can constitute a significantbarrier to the transfer of heat between the fuel and the coolant. Inorder to reduce such gaps, the classical treatment of fuel elementsgenerally involves the application of an isostatic pressure on the fuelelements. However, this is not satisfactory because it requiresexpensive equipment and complicated manipulation.

BRIEF SUMMARY OF THE INVENTION The present invention has for its objectsa new method that is intended to reduce or eliminate any gaps which mayexist between the casing and the nuclear fuel in a fuel element.

The method according to the invention consists in applying a pressure tothe canning of a fuel element through the instrumentality of a plasticmaterial. This method permits the can to be spaced very closely againstthe fuel by the use of simple means that are not cumbersome and whichrequire only elementary equipment.

The method according to the present invention offers in addition theadvantage of enabling the use of a nonisostatic deformation force inorder to obtain very good results, in particular with respect to theimprovement in heat transfer.

The method of the invention has the further advantage that it clamps thecanning closely around the fuel so as to reduce relative movementbetween them. This constitutes an important safety factor for nuclearreactors, and especially for fast breeders.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE INVENTION FIGURE 1 is across sectional view of a typical type of fuel element to which thisinvention is applicable.

FIGURES 2 through 5 are fragmentary cross sectional views of a fuelelement as it goes through the sequence of steps taught by thisinvention.

DETAILED DESCRIPTION Referring now to FIGURE 1, it will be seen thatthis fuel element comprises a head portion 7, a foot portion 6, andintermediate the head and foot is a case 12 (of adequate mechanicalresistance) which contains nuclear fuel 10 (such as the oxides andcarbides of uranium and/or plutonium). The nuclear fuel 10 may be in theform of pellets or in the form of a solid block, and is maintained inthe desired position within the casing 12 by means of plugs 8 and 9. Thecasing 12 may consist of stainless steel, zircalloy, high nickel alloys,niobium and molybdenum alloys, and the like. The preferred thickness isbetween about 0.2 and 0.8 mm.

FIGURE 2 is a cross sectional view which illustrates that even thoughthe casing 112 closely surrounds the nuclear fuel 10, a number of gapscan exist and these gaps do constitute a significant barrier to thetransfer of heat from the nuclear fuel 10 to the casing 12 and thesurrounding cooling medium.

According to the first embodiment of my invention, the arrangement shownin FIGURE 2 is surrounded by an envelope of plastic material 14. Theplastic material may for instance be a polyvinyl chloride resin,polyvinylidene chloride resin, a polystyrene resin, a polyester resin, apolyamide resin, a polyether resin, a polyurethane resin, or the like,and although the thickness of the plastic envelope is not critical, athickness of between 1 and 2 millimeters is preferred. Non-isostaticdeformation forces, such as forging (hammering, rotary swaging) maythereafter be applied to plastic envelope 14 and, since the plasticenvelope is not very compressible, the applied deformation force will betransmitted rather uni formly to the casing. After this mechanicaltreatment, the envelope of plastic material is removed from around thecasing 12 (e.g. by stripping) and the thus-treated fuel element wouldappear as is shown in FIGURE 5, containing practically no gaps, as caneasily be seen by contrasting FIGURES 2 and 3.

It will be obvious to those skilled in this art that it is possible toenvelope the casing 12 with synthetic resin in a number of differentways. For example, if the synthetic plastic material is in tubular formit can be simply slipped over the casing 12. Alternatively, strips ofplastic material could be wound helically around the casing 12. It isalso conceivable that the casing could be dipped one or more times inmolten plastic or sprayed with liquid plastic which would thereaftersolidify.

According to a second embodiment of our invention, the fuel element inthe form shown in FIGURE 2, (containing gaps 8) is first surrounded byan envelope of plastic material 14 and then further surrounded by themetallic tube 16 (made of iron, copper, alloys, or the like and about .2mm. thick) as is shown in FIGURE 4. This assembly is then subjected tothe same sort of mechanical deformation treatment (such as forging,hammering, rotary swaging) as was previously described, and the applied,localized deformation forces are transmitted uniformly tothe casing 12through the plastic envelope 14 to thus reduce or eliminate gaps.Following this mechanical treatment, the metallic tube 16 and theplastic material 14 are removed (as by stripping or cutting) and theresulting fuel element will appear as is shown in FIGURE 5, havingeither no gaps or a greatly reduced number of gaps.

In conclusion, while the foregoing specification and drawing describethe construction, of preferred embodiments of the instant invention, itis to be understood that ave do not intend to limit ourselves to theprecise con- :tructions and arrangements herein disclosed, since thevarious details of construction, form and arrangement nay obviously bevaried to a considerable extent by tnyone skilled in the art withoutreally departing from :he basic principles and novel teachings of thisinvention 1nd without sacrificing any of the advantages of the in-Iention, and accordingly it is intended to encompass all :hanges,variations, modifications and equivalents falling within the scope ofthe appended claims.

What is claimed is:

:1. A method for reducing any gaps between the easng and the fuelmaterial in a nuclear fuel element which :omprises:

(a) surrounding the casing with an envelope of synthetic plasticmaterial,

(b) applying a non-isostatic deformation force to the assembly set forthin (a), and

(c) removing the envelope of synthetic plastic material.

2. A method according to claim 1, wherein the applied leformationpressure is non-isostatic pressure.

3. A method according to claim 11, wherein said dei'ormation pressure isexerted by forging.

4. A method according to claim 1, wherein the de- Formation force isapplied by hammering.

5. A method according to claim 1, wherein said pres- .ure is applied byrotary swaging.

6. A method according to claim 1, wherein said synthetic plasticmaterial is selected from the group consisting of polyvinylidenechloride resins, polyvinyl chloride resins, polystyrene resins,polyester resins, polyamide resins, polyetherresins and polyurethaneresins.

7. A claim according to claim 1, wherein the envelope of syntheticplastic material is in turn surrounded by a metallic envelope prior tothe application of the deformation force.

8. A claim according to claim 1, wherein the thickness of said envelopeof synthetic plastic material is between about 1 and 2 mm.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 3,100,742 8/1963 McGeary et al 29480 3,291,870 12/1966 Allison 29--423 3,089,830 5/1963 McGeary et a1 29-445 3,192,6217/1965 Bauer et a1. 2942 X THOMAS H. EAGER, Primary Examiner US. Cl.X.R. 29-423, 445

